1. Field of the Invention
The present invention relates to a driving system, and particularly to a cold cathode fluorescent lamp (CCFL) driving system that can be utilized to digitally drive a CCFL load included in the system.
2. General Background
Fluorescent lamps are typically used in a number of applications where light is required but power available to generate the light is limited. One such application is a backlighting for a notebook computer or a similar portable electronic device. One popular type of the fluorescent lamp is cold cathode fluorescent lamps (CCFLs), which are almost universally used in panels of various LCDs (liquid crystal displays). The CCFLs require a high starting voltage (on the order of 700-1,600 volts) for a short period of time, to ionize gas contained within the CCFL tubes and ignite the CCFLs. After the gas in the CCFLs is ionized and the CCFLs are lit, less voltage is needed to keep the CCFLs on.
A CCFL tube typically contains a gas, such as Argon, Xenon or the like, along with a small amount of Mercury. After an initial ignition stage and the formation of plasma, electrical current flows through the CCFL tube, which results in the generation of ultraviolet light. The ultraviolet light in turn irradiates a phosphoric material coated on the inner wall of the CCFL tube, resulting in the emission of visible light. This process is generally achieved by the application of a driving system that can be utilized to generate an AC voltage to drive the CCFL when a DC voltage is initially applied.
FIG. 3 shows a conventional CCFL driving system 10. The system 10 broadly includes a power source 12, a CCFL driving circuit 16, a controller 14, a feedback loop 18, and a CCFL load 11. The power source 12 supplies a DC voltage to the CCFL driving circuit 16 under the control of the controller 14, thus generating an AC voltage to the CCFL load 11 through the CCFL driving circuit 16. The CCFL driving circuit 16 is typically a self-oscillating DC to AC converter. Generally, the CCFL driving circuit 16 includes a transformer 161 having a primary winding and a secondary winding, a first and a second switches S1, S2, and a drive circuit 163. The power source 12 and the switches S1, S2 are coupled to the primary winding of the transformer 161. The drive circuit 163 is coupled to the first and second switches S1, S2 to alternately drive the first and second switches S1, S2 to conduct. Accordingly, two paths are defined by the first and second switches S1, S2, with the first switch S1 defining a first conducting path and the second switch S2 defining a second conducting path. The CCFL load 11 includes one or more lamps CCFL1, CCFL2, and is coupled to the secondary winding of the transformer 161 through a capacitor Co. The feedback loop 18 typically includes a sense resistor Rs, which provides a feed back (FB) signal indicative of the current flowing through the CCFL load 11 to the controller 14. The controller 14 typically includes a comparator 15 and a pulse width modulator 17 coupled in series. The comparator 15 is provided to receive a reference signal REF and the FB signal. The comparator 15 then produces a control signal CN corresponding to a comparison of the FB signal and the reference signal, to control the pulse width modulator 17. The pulse width modulator 17 generates a pulse width modulated signal, based at least on the control signal, to the drive circuit 163 to alternately control the first and second switches S1, S2 to conduct. Thus the power delivered to the CCFL load 11 is regulated, which will be discussed in detail below.
FIG. 4 provides a signal representation of the pulse width modulator 17. The pulse width modulator 17 generates a pulse width modulated (PWM) signal 36 typically set by an oscillator 22 and a variable selector 24. The variable selector 24 is provided to adjustably set a pulse width ‘L’ of the PWM signal 36, and thus permit an appropriate amount of power to be delivered to the CCFL load 11. The variable selector 24 varies the value of a DC signal 30, i.e. the power delivered to the CCFL load 11, and determines a desired dim setting by increasing or decreasing the DC signal 30. The oscillator 22 is provided to generate a triangular waveform 34 of predetermined frequency, as an input to the pulse width modulator 17. The DC signal 30 generated is superimposed upon the triangular waveform 34. As illustrated in FIG. 4, a section is defined by the intersections of the DC voltage 30 with each of the rises 25a and falls 25b of each triangular wave 34. The section determines the leading and falling edges of each pulse, and thereby the pulse width ‘L’ of the PWM signal 36. Thus a higher value of the DC signal 30 generates a smaller pulse width ‘L’, and a lower value of the DC signal 30 generates a larger pulse width ‘L’. Alternatively, a section defined by each falling edge 25b and the next rising edge 25c is utilized to generate the pulse width ‘L’. Therefore, the pulse width modulator 17 provided in the controller 14 is actuated to increase or decrease the DC signal 30 by the control signal CN. The PWM signal 36 thereby generated is applied to the drive circuit 163 to drive the first and second switches S1, S2 to alternately conduct, thus regulating power delivered to the CCFL load 11.
The system 10 described herein is generally composed of separate, large components, which occupy much valuable “real estate” on a supporting substrate such as a circuit board. Circuit designers generally require modern integrated circuits to be contained in very small packages. In addition, the separate, large components add to the complexity and cost of the overall design and manufacturing of the whole driving system. Furthermore, the system 10 is basically designed to provide a single operation/function with respect to the CCFL load 11. However, modern digital technology trends dictate that various kinds of circuits, including driving circuits, should not be limited to performing one or a handful of relatively precise and/or complex operations. Rather, the circuits should be able to perform a combination of operations when they are connected to a computer. For example, a plurality of CCFLs or a selected combination of CCFLs may be required to operate in a predetermined or controllable order during intermittent or particular periods, or continuously. To accomplish such processes, control by a computer is required. Yet the system 10 has a little capability to be compatible with a computer in order to accomplish such processes.
Other similar CCFL driving systems can be found in U.S. Pat. Nos. 6,501,234 and 6,396,722, and Taiwan Pat. Publication Nos. 423,204, 502,928 and 485,701. Each of these patents is incorporated herein by reference as disclosing a circuit of a type similar to that shown in FIG. 3. Each of the disclosed circuits may be subject to drawbacks similar and/or additional to the drawbacks detailed above in relation to the system 10 shown in FIG. 3.
What is needed, therefore, is a CCFL driving system that can be utilized to digitally drive a CCFL load.
What is also needed is a CCFL driving system partly integrated into a chipset in order to reduce manufacturing costs.
What is further needed is a CCFL driving system that includes a controller therein and permits the controller to be integrated within a chipset, thus providing a small overall package.